Heat-sensitive copying-paper



Nov. 12, 1957 CLARK 2,813,043

HEAT-SENSITIVE COPYING-PAPER Filed larch 28, 1955 .1- \\\\\\\i\ /6 s esY///////smq// /Z 7- I N VEN r02 BRYCE L. CLARK 6 I V ATTOP/VEY5 United States Patent Office 2,813,043 Patented Nov. 12, 1957 HEAT-SENSITIVE COPYDIG-PAPER Bryce L. Clark, White Bear Lake, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn, a corporation of Delaware Application March 28, 1955, Serial No. 497,171

2 Claims. (Cl. 11736) This invention relates to novel duplicator sheet material in the nature of copying-paper useful in preparing copies of printed matter or the like by ther'mographic copyingmethods.

The copying-method employed involves intensive irradiation of the printed original with radiant energy which is preferentially absorbed and converted to heat energy by the printed or colored areas of the original, and utilization of the heat-pattern thus formed in bringing about a corresponding visible change in a heat-sensitive duplicator sheet material. With suitable sources of radiant energy and other conditions, the method is adapted to the reproduction of typewritten or printed material, penciled notes or sketches, pictures, drawings, and other graphic subjectmatter; the significant requirement being the selective absorption and conversion to heat of the intense radiant energy in the areas of the printed surface delineating the subject-matter to be reproduced.

The method has been described in application Serial No. 747,338 of Carl S. Miller, filed May 10, 1947; and useful heat-sensitive copying-papers have been described in Miller et al. Patents Nos. 2,663,654-7. These copyingpapers employ as the heat-sensitive material various coinbinations of stable ionizable dissociable reactant materials capable of visibly reacting together under defined conditions. The teachings of these patents are made a part hereof by reference.

The present invention provides a heatsensitive copyingpaper in which the chemically reactive heat-sensitive layer is enclosed between a transparent carrier sheet and a visibly opaque outer protective coating. There is provided a sheet material having high sensitivity in the copying-process while being highly resistant to abrasion and moisture vapor and providing for copies of greatly improved contrast.

The copying-paper of the present invention is well adapted to the reproduction of printed pages of books, typewritten letters, etc. by back-printing. In this process the copying-paper is held with the transparent carrier sheet in heat-conductive contact with the reverse surface of the thin printed original. Suitably irradiating the printed surface then causes the formation of a duplicate visible image in the heat-sensitive layer of the copyingpaper, which is viewed through the transparent carrier.

My novel copying-paper is also well adapted to frontprinting operations. In this process the copying-paper is held against the printed surface, with the visibly opaque protective coating in heat-conductive contact therewith, and the radiant energy is applied through the copyingpaper. Here the sheet must permit the passage of the radiant energy employed, without darkening or other deleterious effects. Since the heat evolved at the irradiated printed surface need penetrate only the thin protective coating of the copying-paper rather than the paper or other backing of the graphic original, the front-printing process is particularly well adapted to the reproduction of originals printed on thick paper or on other carriers having poor heat transfer properties.

The front-printing process is i lustrated schematically in the accompanying drawing, showing in perspective a portion of a sheet of copying-paper 10 in isolated heatconductive contact with the printed surface of the printed original 12 having radiation-absorptive printed areas 13 on a substantially less radiation-absorptive background surface 14. The copying-paper consists of a thin transparent carrier sheet 15, a visibly heat-sensitive chemically reactive layer 16, and a visibly opaque surface layer 17. The copying-paper is held in heat-conductive pressurecontact with the surface of the original. The copyingpaper transmits the radiation from source 13 to the surface of the original. Radiation falling on the unprinted and non-absorptive area 14 is diffused, reflected or transmitted unchanged, Whereas radiation falling on the printed areas 13 is absorbed and converted to heat energy. The heat evolved flows through the interface to the heatsensitive layer 16 where it makes possible a chemical reaction between the chemically reactive components of such layer, resulting in the formation of the visible reproduction 1) of the original printed character 13, here indicated to be the letter M. The reproduction is visible through the transparent carrier layer 15 and exhibits a high degree of contrast against the visibly opaque background layer 17.

The invention will now be further described and illustrated in terms of an exemplary but non-limitative formulation and structure, in which all proportions are given in parts by weight unless otherwise specified.

The thin transparent carrier sheet is commercially available map overlay tracing paper, a transparent substantially non-porous lightly calendered paper having a basis Weight of 25 lbs. per ream (24 x 36/500). Other equivalent paper or film backings, e. g., flax tissue, cellophane, or parchment paper are useful but somewhat less desirable because of curling or other problems.

The paper is first coated with a smooth uniform layer of a fluid dispersion, in a binder solution, of interreactive chemical components in particulate form. The amount of dispersion applied is sufiicient to provide a dried residue of approximately 0.07 lb. per sq. yd. The coating is dried at normal room temperature. The dispersion is prepared in accordance with the disclosure provided in Example 1 of Miller et al. Patent No. 2,663,657 and contains approximately parts by Weight of ferric stearate, 35 parts of alcohol-insoluble precipitate of hexa methylenetetramine and pyrogallic acid, and 30 parts of polyvinyl butyral, together With 4 parts of oxalic acid if desired, in an amount of alcohol sufiicient to provide a coatable mixture. The reactive components will react together at room temperature in the presence of a mutual solvent, such as benzol, capable of permitting ionization of the components, but do not react in the solid form either in the dispersion or in the dried coating. However, on heating the coating to or somewhat above the melting temperature of one of the componentsin this case the ferric stearatea reaction occurs, resulting in a visible change as already indicated.

A further coating is next applied over the heat-sensitive layer, in this case of a heavily pigmented solution of a soluble binder. The composition contains 11.90% titanium dioxide pigment (Titanox A), 7.35% ethyl cellulose binder, and 80.75% acetone. The weight of the dry residue is approximately 0.085 lb. per sq. yd., which is suflicient to provide the desired visual opacity while still permitting transmission of the high intensity radiation employed in copying.

The coated sheet material is placed in contact with the printed surface of a typewritten or printed page which is then briefly and intensively irradiated as indicated in the drawing. Useful irradiation is obtained with a 3000- watt tubular lamp having a coiled tungsten filament 10 inches in length; a still more effective source employs a General Electric T3 lamp having a coiled line filament within a inch diameter quartz tube and operated at 280 volts. The 10-inch filament draws 1350 watts and provides a color temperature of about 2800 K. The radiation is concentrated on a narrow line by a suitable reflector and the line moves across the area to be treated in order to provide the required brief intense radiation. The intensity of irradiation is sufficient to cause charring of the sheet if maintained for more than about one second on the same area.

A visible change may also be produced in the copyingpaper prepared as just described by pressing heated metal type or similar source of heat against the sheet. By this means it may be determined that the visible change occurs at temperatures above approximately 80 (3., which corresponds reasonably closely to the melting temperature of the ferric stearate employed. Compounds and compositions are also useful which are activated at other temperatures within the approximate range of 601ZG C. At much lower activation temperatures the sheet is not sufficiently stable on storage, since storage temperatures may at times approach such temperatures. At much higher temperatures softening of the binder, or degradation of the paper backing or of the original, may be experienced; furthermore the attainment of such unduly elevated temperatures by readily available means is difficult.

Tungsten filament radiation sources produce radiant energy which is largely in the infra-red range. Equally effective results, in terms of the ultimate copy, may be obtained with radiation containing very little or no infra red, such as the radiation from known monochromatic light sources or obtained by selective absorption of portions of broad bands of irradiation. Visible light is particularly effective since its use permits the copying of originals which normally are non-absorptive of infra-red. However, the required high intensity of irradiation is ordinarily more readily obtainable from sources producing a considerable proportion of infra-red. Furthermore most books, letters, and other documents of which copies are desired are ordinarily printed with inks employing infra-red-absorptive pigments such as lampblack. Hence the radiation employed with my novel heat-sensitive copying-paper will ordinarily be rich in infra-red and the copying-paper will therefore be required primarily to be capable of transmitting such radiation without visible change in the copying-paper itself.

Surprisingly, the high pigment loading in the protective surface coating of the foregoing example permitsthe passage of the infra-red radiation and at least a substantial portion of the visible radiation. The radiation-absorptive areas of the original are strongly heated by the radiation passing through the copying-paper. The copying-paper remains unchanged when irradiated by itself. Nevertheless the coating appears highly opaque, and forms an intense white background for the deeply colored copy 4 obtained in the heat-sensitive layer during the copying process. The completed copy is quite similar in ap pearance to a typewritten original on letter-paper, rather than resembling the semi-transparent products obtained in the absence of the opacifying layer.

A further important advantage of the novel structure here disclosed is in the increased stability andimproved handling characteristics of the sheet material.

In a further example, a chemically reactive heat-sensitive layer was used which was converted from a faint tan to an intense blue color when reacted, and this layer was covered with a visibly opaque outer protective layer having an intense red color produced by the incorporation of India Red Toner pigment.

Other pigments which have been found useful are zinc oxide, zinc sulfide, antimony trioxide, lead carbonate. Extenders such as barium sulfate, barium carbonate, calcium carbonate and magnesium carbonate may be added. Other opacifying agents may be substituted which are effective in providing a visually opaque but radiationtransmitting outer layer. Likewise other binders may be substituted for the ethyl cellulose. Polyvinyl butyral is typical. The binder mixture should obviously contain no solvent or reactant which might penetrate the chemically reactive. heat-sensitive layer and activate the chemical reaction producing the visible change. After the outer layer is applied, the chemically reactive heat-sensitive layer is protected both physically against abrasion and chemically against activation by solvents.

, What is claimed is as follows:

1. A stable, moisture-resistant, heat-sensitive copyingpaper capable of providing high-contrast copies of graphic subiect-matter by thermographic copying processes as herein described, consisting of a thin, transparent, flexible backing, an intermediate chemically reactive, visibly heatsensitive coating, the chemical reaction occurring within said coating on heating said sheet being responsible for the visible change, and a visibly opaque protective surface coating providing strong contrast for visible copy produced in said intermediate coating.

2. A stable, moisture-resistant, heat-sensitive copyingpaper capable of providing high-contrast copies of graphic subject-matter by thermographic processes as herein described, consisting of a thin, transparent, flexible paper backing, an intermediate visibly heat-sensitive coating which is chemically reactive to form a visibly differently colored reaction product on heating the copying-paper, and a visibly oqaque, infra-red-transmissive, protective surface coating comprising a binder and a pigment having a color different from the color of said colored reaction product, said surface coating providing strong contrast for visible copy produced in said intermediate coating.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A STABLE, MOISTURE-RESISTANT, HEAT-SENSITIVE COPYINGPAPER CAPABLE OF PROVIDING HIGH-CONTRAST COPIES OF GRAPHIC SUBJECTED-MATTER BY THERMOGRAPHIC COPYING PROCESSES AS HEREIN DESCRIBED, CONSISTING OF A THIN, TRANSPARENT, FLEXIBLE BACKING, AN INTERMEDIATE CHEMICALLY REACTIVE, VISIBLY HEATSENSITIVE COATING, THE CHEMICALLY REACTION OCCURING WITHIN SAID COATING ON HEATING SAID SHEET BEING RESPONSIBLE FOR THE VISIBLE CHANGE, AND A VISIBLY OPAQUE PROTECTIVE SURFACE COATING PROVIDING STRONG CONTRAST FOR VISIBLE COPY PRODUCED IN SAID INTERMEDIATE COATING. 